Iron-base oxide dispersion strengthened alloys (iron-base ODS alloys) have been developed for high temperature applications. Chromium and aluminum is typically added to the iron-base for resistance to oxidation, carburization and hot corrosion. The alloy is strengthened with an oxide stable at high temperature, such as a yttrium oxide. The oxide is uniformly distributed throughout the alloy as a finely distributed dispersoid. Iron-base ODS alloys in the form of sheet are particularly useful for gas-turbine combustion chambers, components of advanced energy-conversion systems and high temperature vacuum furnaces.
The properties of sheet of iron-base ODS alloys are extremely process dependent and process sensitive. Ideally, iron-base ODS sheet is formed having a pancake-shaped grain structure. The pancake-shaped grain structure contributes to isotropic properties in both the longitudinal and transverse properties of sheet. For purposes of this invention, transverse direction refers to transverse to the last rolling direction and longitudinal refers to in the rolling direction. For turbine engine parts it is often critical to have substantially uniform properties in the transverse and longitudinal directions. The problem with processing iron-base ODS sheet is that the properties are direction dependent. When material is processed in only one direction, the properties in the longitudinal direction are disproportionately increased in relation to the transverse direction.
To date, iron-base ODS alloys have been produced by a low yield multi-step process. First, the alloy was prepared by ball milling powder. The powder was then encased in steel cladding to form a billet. The billet was extruded and hot rolled. A pickling operation was then used to remove the can. To finish the sheet, the sheet was then cold rolled to final size. Cold rolling at temperatures slightly above room temperature may be required because iron-base ODS alloys often have a high ductile-to-brittle transition temperature. At these cold rolling temperatures very little, if any, recrystallization occurs.
In order to optimize the isotropic properties in the transverse and longitudinal directions, iron-base ODS alloys are cross-rolled. The cross-rolling is accomplished by rolling in a first direction, rotating the sheet 90.degree. and rolling. After each significant reduction in thickness, the sheet is rotated 90.degree.. To accomplish these multiple turns, a hand-mill is used to roll the sheet down to finished size. The problem with these hand-mill operations is the process is limited by width of the mill each time a sheet is rotated 90.degree.. In addition, this process is labor intensive and has produced maximum product yields of only 25%.
It is an object of the invention to provide a more continuous and automated method for producing iron-base ODS sheet having predetermined properties in the transverse and longitudinal directions.
It is further object of the invention to increase production yields.
It is a further object of the invention to produce ODS sheet of larger width and length having improved mechanical properties.